The present invention relates to a method of imaging a living body tomogram by utilizing a nuclear magnetic resonance phenomenon, and more particularly to a method of imaging a status in a body which varies from time to time. The present invention is useful for medical diagnosis.
Dynamics study for tracing time-sequential changes of a test subject has been normally done in a field of X-ray CT ("Whole-Body X-ray CT Scanner TCT-900S" by Issei Mori et al, TOSHIBA REVIEW, Vol. 42, No. 2, 1987, pages 80-82). In the X-ray CT, a minimum imaging unit capable of generating one image is continuously scanned a plurality of times so that a certain length of measurement data is obtained. There is no discrimination between data with respect to value of data and all data are homogeneous. Thus, one complete image can be generated from a portion of the data corresponding to one scan data extracted from any position in the measurement data between one scan to the next scan. By utilizing the above nature, it is possible to reduce a scan time, from a time actually required for one scan. For example, if one-second scan is continuously effected 30 times and a series of data portions each corresponding to one scan data are extracted from the measurement data while the data portions stagger from each other by 0.3-second area, approximately 100 sheets of images of successive scans started at an interval of 0.3 second are obtained. Thus, the time resolution is improved from one second to 0.3 second.
In a field of magnetic resonance imaging (MRI) which utilizes the nuclear magnetic resonance phenomenon, clinical experiment for the dynamic study has been done ("Dynamic MRI of Small Liver Hemangioma" NMR MEDICINE Vol. 6, Supplement-2, 1986, page (58). However, the method for improving the time resolution used in the X-ray CT is not used by the reason described afterward, and one image is simply generated from each scan.
The X-ray CT makes an image of distribution of X-ray absorptivity while the MRI makes an image of distribution of hydrogen. Accordingly, the images generated thereby relate to different subjects and it is not possible to use the X-ray CT in place of the MRI. Further, the X-ray CT involves a risk of radiation ray problem. Since the MRI takes approximately two minutes for one ordinary imaging, it is possible to observe the status at an interval of approximately two minutes. If a high speed imaging technique which shortens a repetition time is used, one imaging cycle will complete in approximately five seconds. Even in such a case, the time resolution is five seconds which is farely poor compared to that of the X-ray CT to which the method for improving the time resolution is applicable.
In the MRI, the method for improving the time resolution which is used in the X-ray CT cannot be used because the MRI is operated in a different principle than the X-ray CT and the measurement data in the MRI lacks homogenity which the measurement data in the X-ray CT has. The scan in the MRI is conducted by changing a strength of a phase encoded pulse (306 in FIG. 4) for each measurement. FIG. 2 shows an example of application sequence of the phase encoded pulses in a related art. The strength of the phase encoded pulse is monotoneously and constantly increased from a negative maximum to a positive maximum, and one measurement is done at each strength level.
The n strength levels of the phase encoded pulses are discriminated by phase encoded numbers sequentially assigned in the order of strength, and the minimum (negative maximum) strength level is assigned with zero. Thus, the strength G.sub.p (i) of the phase encoded pulse having the phase encoded number 0 is given by EQU G.sub.p (i)=2G.sub.pmax.i/n-G.sub.pmax
where G.sub.pmax is the maximum amplitude of the phase encoded pulse, and i is an integer from 0 to n-1. In the related art, the application sequence of the phase encoded pulses is changed by monotoneously increasing (from 0 to n-1) or decreasing (from n-1 to 0) the phase encoded number i with time. FIG. 2A shows a simplified graph for the former case. Usually, n=256.
As shown in FIG. 2A, the scan with stepwise change of the strength of the phase encoded pulse from 0 to n-1 with time is sequentially effected three times, and a series of data portions each corresponding to one scan data which is staggered from others by 1/3 scan area are extracted from the measurement data in accordance with the method for improving the time resolution used in the X-ray CT, and images are generated from the respective data portions. FIG. 2B schematically shows the above process. A group of lines 404, 405 and 406 which are collectively designated by 401 show ranges of one-scan data of the measurement data 400, each of which is used to generate one image. FIG. 2C shows one-scan measurement data. As shown in FIG. 2C, a measurement signal is sampled in a measurement space. The number of times of sampling is usually 256. The measurement signals for the phase encoded pulses 0 to n-1 are transformed by two-Dimension Fourier Transform (2DFT) to obtain a tomogram image. In general, when the strength of the phase encoded pulse is high, a high frequency component of the image is measured. This area is designated by numeral 402. On the other hand, when the strength is low, a low frequency component of the image is measured. This area is designated by numeral 403.
The strength of the phase encoded pulse of FIG. 2A stepwisely changes from the negative maximum to the positive maximum. Thus, at an early stage of the scan, the high frequency component 402 of the image is measured, at a middle stage of the scan, the low frequency component 403 is measured, and at a late stage of the scan, the high frequency component 402 is again measured.
In the X-ray CT, the scan is effected by rotating an imaging mechanism around a test subject. Thus, the scans starting from any initial angles are equivalent. In this sense, the measurement data is homogeneous, and the above method for extracting the one-scan data from the continuous data with each one-scan data staggering from others is successful. On the other hand, in the MRI, the scan is effected by changing the strength of the phase encoded pulse and the strength of the phase encoded pulse relates to the frequency component of the generated image, as shown in FIG. 2C. As a result, the measurement data in the MRI lacks the homogeneity. In FIG. 2B, the image generated from the data in the range 405 has the high frequency component thereof updated from .circle.1 to .circle.1 ' compared to the high frequency component 402 of the image generated from the data in the range 404. The image generated from the data in the range 406 has the low frequency component thereof updated from .circle.2 to .circle.2 ' compared to the low frequency component 403 of the image generated from the data in the range 405. Since the high frequency component and the low frequency component are separately updated, the continuity of the series of images is very poor.